British Pat. No. 1,600,556 relates to a touch-sensitive key having a radiation-propagating unit, such as an optical prism, arranged between a radiation source, such as a light-emitting diode, and a radiation detector such as a phototransistor. When an outer surface of the radiation-propagating unit is touched, the conditions for total internal reflection in this surface are changed, resulting in an identifiable change in the radiation from the radiation source which reaches the radiation detector. Incident external radiation does not reach the radiation detector because it is always internally reflected within the said unit so as to leave through a surface remote from the radiation detector.
This arrangement is generally satisfactory. However, practical problems can arise if the key is used with an optical system under water. The refractive index inside the radiation-propagating unit has to be above a certain critical value to ensure ray separation between detector rays (i.e. rays from the radiation source to the radiation detector) and external rays. This critical value is 1/.sqroot.2 where the medium outside the prism is air and about 1.66 where the medium is water. However, a refractive index of the order of 1.7 cannot be readily achieved with inexpensive materials such as ordinary glass or plexiglass, and expensive heavy optical glass may be necessary.